Rail fasteners of the general type described above have heretofore been proposed: see e.g., German Auslegeschrift No. 1204697 and U.S. Pat. Nos. 3,576,293 and 3,784,097. The assemblies of each of the foregoing references include a base plate, a rail plate for attachment to the rail and positioned between the rail and the base plate, and an elastomeric material such as neoprene interposed between the base plate and the rail plate. The elastomeric material supports the rail plate from the base plate, damps vibration of the rail plate and electrically insulates the base plate and the rail plate and/or rail.
Because rapid transit rails are used as electrical conductors for traction power current as well as for train speed command signals, it is necessary to provide and maintain electrical insulation between the rails and the rail support structure. The aforesaid rail fasteners presently in use do provide some electrical insulation between the rails and the rail support structure. However, heretofore, the surface creepage paths provided by the insulating elements of existing apparatus were found to be relatively short and easily contaminated with dirt and wheel and rail wear products. When these contaminated surfaces then became wet by fog, rain, or ground water, electrically conductive paths are formed over which electrical leakage currents flowed. Such leakage currents caused corrosion of the rail as well as of the metal parts of the rail fasteners and supports, resulting in further contamination of the surface creepage paths. This additional contamination of the creepage paths results in further reduction of the electrical resistance of the creepage paths and thence results in larger magnitudes of leakage currents.
Such excessive leakage currents from train operation over poorly insulated rails caused destructive corrosion of rail, rail fasteners, rail support structures, metal tunnel liners, concrete reinforcement bar and other metallic structures. An excessively low rail to rail support structure electrical resistance caused by such corrosion also tended to short out train speed command signals between the rails. In such a situation, the shorted section of track then appears to the train speed command system as though it were occupied by a train, and for safety reasons train operation is disrupted. Such an occurence is generally referred to in the art as a "ghost train".
To prevent loss of electrical train speed command signals and leakage of electrical traction currents over potential creepage paths, rail circuit insulation integrity must be maintained, and the rail fastener or support insulation must provide electrical insulation even when wet and contaminated with electrolyte. Therefore, the rail must be insulated from the supporting structure and/or portions of the rail fastener with electrical insulation means which provides relatively high electrical resistive surface creepage paths. Such high electrical resistive creepage paths will maximize the electrical leakage path resistance between rail and rail support even when the rail and the rail fastener or support apparatus are wet and/or contaminated with electrolyte.
In addition, the rail support insulating device must not interfere with the rail fastener's ability to securely fasten the rail relative to the support structure and to limit relative movement of the rail to within acceptable tolerances in the vertical, lateral, roll and longitudinal directions.
One form of rail support insulating device to prevent the above decribed current leakage is disclosed in U.S. Pat. No. 4,615,484 which included an electrically non-conductive skirt member surrounding and extending outward from the rail fastener and over it support structure and a pair of planar disc members of electrically nonconductive material adjacent the bolt heads. Such a device adds extra components to a rail fastener to overcome the problem of current leakage. It is desirous to have a rail fastener that overcomes the problem of current leakage by incorporating additional electrical protection into the rail fastener.